The job of cleaning aluminum (such as cleaning airplanes) offers problems that are not associated with cleaning other metals. This is because, on the one hand, aluminum is a very reactive metal so that the unprotected metal is subject to severe reactions with cleaning agents and on the other hand, aluminum oxide forming on the surface of the aluminum is typically very hard and tenacious. The result is that when cleaning agents such as those containing sodium and potssium are used, severe etching can occur in places where the cleaning agent has penetrated the oxide layer.
Much effort has been devoted to developing effective methods for cleaning aluminum and these efforts have been generally directed toward the use of aqueous solutions that contain agents that react with exposed aluminum metal to form a coating that prevents corrosion (etching) discoloration and/or loss of adhesion of paint applied subsequent to the cleaning process. Other efforts have been directed toward the use of stabilizing agents that hold benign abrasive particulates in suspension for use as scouring media.
U.S. Pat. No. 4,432,846 to Honeycutt discloses a treatment for aluminum capacitor foil including an acid etch followed by immersion in a passivating carboxyl dip.
U.S. Pat. No. 5,32,5 14 to Sciver et al discloses a corrosion inhibiting cleaning system for aluminum surfaces including blasting the surface with a solution containing alkali metal bicarbonate particulates and a silicate.
U.S. Pat. No. 5,279,755 to Choy et al discloses a thickening aqueous abrasive cleaner with improved colloidal stability. The solution contains fine aluminum oxide particulates that are held in colloidal suspension by appropriate additions of a fatty acid derivative to provide an abrasive cleaner for an aluminum surface.
U.S. Pat. No. 5,346,641 to Argo et al discloses a thickened aqueous scouring cleanser containing an abrasive particulate, bleach and a multivalent buffering agent to confer an effective viscosity. Preferred abrasives include aluminum oxide and calcium carbonate. A preferred bleach is sodium hypochlorite. Preferred buffering agents are divalent phosphates or silicates.
U.S. Pat. No. 5,380,468 to Gober et al discloses an alkaline cleaning solution containing a surfactant, metal base and complexing agent. The preferred surfactant is quaternary ammonium cationic surfactant. The preferred base may be alkali or alkali earth borates, carbonates, hydroxides, silicates.
One of the problems with the solutions and cleaning systems disclosed in the foregoing paragraphs is disposal of the "environmentaly unfriendly" waste solutions containing surfactants, bleaches, etc. that issue from the cleaning operations. Another problem is the necessity to control the compositions of the respective solutions within a concentraion range that is effective for their intended purpose yet do not result in contaminating the surfaces that they are intended to clean.